1. Field of the Invention
The present invention relates to a boring method for a circuit board.
2. Description of Related Art
A flexible printed circuit board is employed to make the electronic devices lighter in weight, thinner and shorter. If the flexible printed circuit board is employed, the line for transferring an electrical signal can be arranged at higher density than using the wire. Therefore, the flexible printed circuit board is increasingly employed as an electrical coupling member to produce the electronic devices that are made lighter in weight, thinner and shorter.
With the miniaturization of electronic devices, it is important that the pattern is formed at high precision and fineness in the flexible printed circuit board (hereinafter abbreviated as a circuit board). As the number of signal lines increases, the circuit board is required to be multi-layered. Thus, various methods have been proposed to effect the interlayer connection of multi-layered circuit board.
A method for making the interlayer connection of the multi-layered circuit board generally employs the through-hole plating. In the through-hole plating, a through hole is formed in the circuit board, and the inner face of the through hole is plated. The through hole for the through hole plating is formed employing a working apparatus having a boring machine such as a drill machine, a punching machine, or a laser beam machine.
The working apparatus has an alignment function for aligning the boring position of the through hole in the circuit board. Therefore, the working apparatus is provided with a CCD (Charge Coupled Device) camera, a lens system, an illumination system, and a movement mechanism, in addition to the boring machine such as a drill machine, a punching machine, or a laser beam machine.
The alignment involves recognizing an alignment mark attached onto the actual circuit board, and conducting computation to correct an error between the position of through hole in the design data and the boring position of through hole in the actual circuit board, on the basis of the position of recognized alignment mark.
Lately, a working apparatus has been developed having a function of correcting errors in the horizontal direction, vertical direction and the rotational direction, as well as comparing the distance between alignment marks in the actual circuit board with the distance between alignment marks in the design data, and correcting the design data in accordance with the ratio of elongation or contraction if the actual circuit board is elongated or contracted. Such a correction is called as the scaling. The positioning of hole in the actual circuit board is conducted on the basis of the corrected design data.
The alignment of the circuit board is performed at two stages of a panel alignment and a pattern alignment, as described below. The panel alignment involves positioning the circuit board on a table of the working apparatus. Also, the pattern alignment involves positioning the hole in the pattern within the circuit board.
FIG. 7 is a flowchart showing a boring method for the conventional flexible printed circuit board.
As shown in FIG. 7, first of all, a circuit board is carried into the working apparatus (step S11). Then, the panel alignment for positioning the circuit board carried in on the table of the working apparatus is performed (step S12). The panel alignment involves picking up an image of the circuit board using a CCD camera of the working apparatus, detecting the alignment mark attached onto the circuit board on the basis of the obtained image, and positioning the circuit board on the table of the working apparatus with reference to the position of the alignment mark.
Next, the pattern alignment is performed to position the hole in the pattern of the circuit board positioned on the table of the working apparatus (step S13). This pattern alignment involves detecting a plurality of alignment marks attached onto the circuit board, modifying errors between the boring position in the actual circuit board and the boring position in the design data, comparing the distance between alignment marks in the actual circuit board with the distance between alignment marks in the design data, and detecting the presence or absence of elongation or contraction of the actual circuit board. If the circuit board is elongated or contracted, the scaling is performed to correct the design data in accordance with its ratio of elongation or contraction. Thereby, the position of boring the hole in the actual circuit board is corrected.
Thereafter, the circuit board is bored, employing the boring machine of the working apparatus (step S14). Lastly, the circuit board is carried out from the working apparatus (step S15).
A thin material such as a flexible printed circuit board often has a distortion component of elongation or contraction or other distortion components. Therefore, in boring, it is required to correct the position of boring by scaling, on the basis of the design data corresponding to the distortion of the circuit board, as described above.
The circuit board is composed of two kinds of materials including a metal part and a resin part for forming a circuit. In particular, the metal part may be possibly subjected to permanent deformation due to a stress applied in a manufacturing process. If a region subjected to permanent deformation and other region are mixed, the degree of elongation or contraction of the circuit board may vary partly. Also, the circuit board may be elongated or contracted anisotropically, or locally.
In such a case, even if the scaling is uniformly applied on the entire circuit board, it is impossible to correct the position of boring in each region precisely. Therefore, it is necessary to reduce the unit of area for making the pattern alignment. If the unit of area for making the pattern alignment is reduced, the precision of the boring position may be improved, but the number of pattern alignments increases accordingly. As a result, it takes more time to make the pattern alignment, decreasing the production efficiency. In this way, the production efficiency and the positional precision of boring contradict.